Method for the production of butadine from n-butane

ABSTRACT

A process for preparing butadiene, comprising nonoxidatively dehydrogenating n-butane from a stream (a) in a first dehydrogenation zone to obtain stream (b) comprising 1-butene and 2-butene; oxidatively dehydrogenating the 1-butene and 2-butene of (b) in the presence of an oxygenous gas in a second dehydrogenation zone to obtain stream (c) comprising n-butane, butadiene, hydrogen, carbon dioxide, and steam; compressing and cooling (c) to obtain stream (d2) comprising n-butane, butadiene, hydrogen, carbon dioxide, and steam; extractively distilling (d2) into stream (e1) comprising butadiene and stream (e2) comprising n-butane, hydrogen, carbon dioxide, and steam; compressing and cooling (e2) to obtain stream (f1) comprising n-butane and water and stream (f2) comprising n-butane, hydrogen, and carbon dioxide; cooling (f2) to obtain stream (g1) comprising n-butane and stream (g2) comprising carbon dioxide and hydrogen; phase separating water from (f1) to obtain stream (h1) comprising n-butane; and recycling (h1) into the first dehydrogenation zone.

RELATED APPLICATIONS

This application is a national stage application (under 35 U.S.C. § 371)of PCT/EP2005/013105 filed Dec. 7, 2005, which claims benefit of Germanapplication 10 2004 059 356.6 filed Dec. 9, 2004.

The invention relates to a process for preparing butadiene fromn-butane.

Butadiene is an important basic chemical and is used, for example, toprepare synthetic rubbers (butadiene homopolymers,styrene-butadiene-rubber or nitrile rubber) or for preparingthermoplastic terpolymers (acrylonitrile-butadiene-styrene copolymers).Butadiene is also converted to sulfolane, chloroprene and1,4-hexamethylenediamine (via 1,4-dichlorobutene and adiponitrile).Dimerization of butadiene also allows vinylcyclohexene to be generated,which can be dehydrogenated to styrene.

Butadiene can be prepared by thermally cracking (steamcracking)saturated hydrocarbons, in which case naphtha is typically used as theraw material. In the steamcracking of naphtha, a hydrocarbon mixture ofmethane, ethane, ethene, acetylene, propane, propene, propyne, allene,butenes, butadiene, butynes, methylailene, C₆ and higher hydrocarbons isobtained.

A disadvantage of the generation of butadiene in a cracking process isthat larger amounts of undesired coproducts are inevitably obtained.

It is an object of the invention to provide a process for preparingbutadiene from n-butane, in which coproducts are obtained to a minimalextent.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE schematically illustrates the steps of the process describedin the Example.

The object is achieved by a process for preparing butadiene fromn-butane, comprising the steps of

-   A) providing a feed gas stream a comprising n-butane;-   B) feeding the feed gas stream a comprising n-butane into at least    one first dehydrogenation zone and nonoxidatively, catalytically    dehydrogenating n-butane to obtain a gas stream b comprising    n-butane, 1-butene, 2-butene, butadiene and hydrogen, with or    without carbon dioxide and with or without steam;-   C) feeding the gas stream b and an oxygenous gas into at least one    second dehydrogenation zone and oxidatively dehydrogenating 1-butene    and 2-butene to obtain a gas stream c comprising n-butane,    butadiene, hydrogen, carbon dioxide and steam,-   D) compressing in at least one first compression stage and cooling    the gas stream c to obtain at least one condensate stream d1    comprising water and a gas stream d2 comprising n-butane, butadiene,    hydrogen, carbon dioxide and steam,-   E) separating the gas stream d2 by extractive distillation into a    product stream e1 consisting substantially of butadiene and a stream    e2 comprising n-butane, hydrogen, carbon dioxide and steam,-   F) compressing in at least one further compression stage and cooling    the gas stream e2 to obtain at least one condensate stream f1    comprising n-butane and water, and a gas stream f2 comprising    n-butane, hydrogen and carbon dioxide,-   G) cooling the gas stream f2 to obtain a condensate stream g1    comprising n-butane and an offgas stream g2 comprising carbon    dioxide and hydrogen,-   H) removing water from the at least one condensate stream f1 and, if    appropriate, from the condensate stream g1 by phase separation to    obtain at least one recycle stream h1 comprising n-butane and at    least one wastewater stream h2, and recycling the at least one    recycle stream h1 into the first dehydrogenation zone.

The process according to the invention is notable for particularlyeffective utilization of the raw materials. Thus, losses of the n-butaneraw material are minimized by recycling unconverted n-butane into thedehydrogenation. The coupling of nonoxidative catalytic dehydrogenationand oxidative dehydrogenation achieves a high butadiene yield. Comparedto the generation of butadiene by cracking, the process is notable forhigh selectivity. No coproducts are obtained. The complicated removal ofbutadiene from the product gas mixture of the cracking process isdispensed with.

In a first process part, A, a feed gas stream a comprising n-butane isprovided. Typically, the starting raw materials are n-butane-rich gasmixtures such as liquefied petroleum gas (LPG). LPG comprisessubstantially saturated C₂-C₅ hydrocarbons. In addition, it alsocomprises methane and traces of C₆ ⁺ hydrocarbons. The composition ofLPG can vary markedly. Advantageously, the LPG used contains at least10% by weight of butanes.

Alternatively, a refined C₄ stream from crackers or refineries may beused.

In one variant of the process according to the invention, the provisionof the dehydrogenation feed gas stream comprising n-butane comprises thesteps of

-   (A1) providing a liquefied petroleum gas (LPG) stream,-   (A2) removing propane and any methane, ethane and C₅ ⁺ hydrocarbons    (mainly pentanes, additionally hexanes, heptanes, benzene, toluene)    from the LPG stream to obtain a stream comprising butanes (n-butane    and isobutane),-   (A3) removing isobutane from the stream comprising butanes to obtain    the feed gas stream comprising n-butane, and, if desired,    isomerizing the isobutane removed to give an n-butane/isobutane    mixture and recycling the n-butane/isobutane mixture into the    isobutane removal.

Propane and any methane, ethane and C₅ ⁺ hydrocarbons are removed, forexample, in one or more customary rectification columns. For example, ina first column, low boilers (methane, ethane, propane) may be removedoverhead, and, in a second column, high boilers (C₅ ⁺ hydrocarbons) maybe removed at the bottom of the column. A stream comprising butanes(n-butane and isobutane) is obtained, from which isobutane is removed,for example in a customary rectification column. The remaining streamcomprising n-butane is used as the feed gas stream for the downstreambutane dehydrogenation.

The isobutane stream removed can be subjected to an isomerization. Tothis end, the stream comprising isobutane is fed into an isomerizationreactor. The isomerization of isobutane to n-butane may be carried outas described in GB-A 2 018 815. An n-butane/isobutane mixture isobtained and is fed into the n-butane/isobutane separating column.

The isobutane stream removed may also be sent to a further use, forexample for preparing methacrylic acid, polyisobutene or methyltert-butyl ether.

The feed gas stream a, comprising n-butane, comprises generally at least60% by weight of n-butane, preferably at least 90% by weight ofn-butane. In addition, it may also comprise C₁-C₆ hydrocarbons assecondary constituents.

In one process part, B, the feed gas stream comprising n-butane is fedinto a dehydrogenation zone and subjected to a nonoxidative catalyticdehydrogenation. In this dehydrogenation, n-butane is partlydehydrogenated in a dehydrogenation reactor over a dehydrogenatingcatalyst to give 1-butene and 2-butene, and butadiene is also formed. Inaddition, hydrogen and small amounts of low boilers (methane, ethane,ethene, propane and propene) are obtained. Depending on the method ofthe dehydrogenation, CO₂, water and nitrogen may also be present in theproduct gas mixture of the nonoxidative catalytic n-butanedehydrogenation. Unconverted n-butane is additionally present in theproduct gas mixture.

The nonoxidative catalytic n-butane dehydrogenation may be carried outwith or without oxygenous gas as a cofeed. It is preferably carried outas an autothermal nonoxidative catalytic dehydrogenation with feeding ofoxygen as a cofeed. In the autothermal method, the heat required isgenerated directly in the reactor system by combustion of hydrogenand/or hydrocarbons in the presence of oxygen. If appropriate, a cofeedcomprising hydrogen may additionally be admixed. Oxygen may also be fedin as pure oxygen or as oxygenous gas, for example as air. In order torestrict the inert gas fraction, oxygen may be fed in as oxygen-richgas, generally having an oxygen content of at least 75% by volume,preferably at least 90% by volume. A suitable oxygenous gas is oxygen oftechnical grade purity, having an oxygen content of approx. 99% byvolume, As a result of the use of an oxygenous cofeed with a high oxygencontent only small amounts of inert gases (nitrogen) are introduced intothe overall process. This has an advantageous effect in the subsequentworkup, since the losses of C₄ hydrocarbons which are discharged withthe inert gases are lower.

One feature of the nonoxidative method compared to an oxidative methodis that free hydrogen is not formed in the oxidative dehydrogenation.

The nonoxidative catalytic n-butane dehydrogenation may in principle becarried out in any reactor types and methods disclosed by the prior art.A comparatively comprehensive description of dehydrogenation processessuitable in accordance with the invention is also contained in“Catalytica® Studies Division, Oxidative Dehydrogenation and AlternativeDehydrogenation Processes” (Study Number 4192OD, 1993, 430 FergusonDrive, Mountain View, Calif., 94043-5272, USA).

A suitable reactor form is a fixed bed tubular or tube bundle reactor.In these reactors, the catalyst (dehydrogenation catalyst and, whenworking with oxygen as the cofeed, optionally a specialized oxidationcatalyst) is disposed as a fixed bed in a reaction tube or in a bundleof reaction tubes. The reaction tubes are customarily heated indirectlyby the combustion of a gas, for example a hydrocarbon such as methane,in the space surrounding the reaction tubes. It is favorable to applythis indirect form of heating only to about the first 20 to 30% of thelength of the fixed bed and to heat the remaining bed length to therequired reaction temperature by the radiant heat released in the courseof indirect heating. Customary reaction tube internal diameters are fromabout 10 to 15 cm. A typical dehydrogenation tube bundle reactorcomprises from about 300 to 1000 reaction tubes. The internaltemperature in the reaction tubes typically varies in the range from 300to 1200° C., preferably in the range from 500 to 1000° C. The workingpressure is customarily from 0.5 to 8 bar, frequently from 1 to 2 bar,when a small steam dilution is used (analogously to the Linde processfor propane dehydrogenation), or else from 3 to 8 bar when a high steamdilution is used (analogously to the steam active reforming process(STAR process) for dehydrogenating propane or butane of PhillipsPetroleum Co., see U.S. Pat. No. 4,902,849, U.S. Pat. No. 4,996,387 andU.S. Pat. No. 5,389,342). Typical gas hourly space velocities (GHSV) arefrom 500 to 2000 h⁻¹, based on the hydrocarbon used. The catalystgeometry may, for example, be spherical or cylindrical (hollow orsolid).

The nonoxidative catalytic n-butane dehydrogenation may also be carriedout under heterogeneous catalysis in a fluidized bed, as described inChem. Eng. Sci. 1992 b, 47 (9-11) 2313. Appropriately, two fluidizedbeds are operated in parallel, of which one is generally in the state ofregeneration. The working pressure is typically from 1 to 2 bar, thedehydrogenation temperature generally from 550 to 600° C. The heatrequired for the dehydrogenation is introduced into the reaction systemby preheating the dehydrogenation catalyst to the reaction temperature.The admixing of a cofeed comprising oxygen allows the preheater to bedispensed with and the required heat to be generated directly in thereactor system by combustion of hydrogen and/or hydrocarbons in thepresence of oxygen. If appropriate, a hydrogen-containing cofeed mayadditionally be admixed.

The nonoxidative catalytic n-butane dehydrogenation may be carried outin a tray reactor with or without oxygenous gas as a cofeed. It ispreferably carried out with oxygenous gas as a cofeed. This reactorcomprises one or more successive catalyst beds. The number of catalystbeds may be from 1 to 20, advantageously from 1 to 6, preferably from 1to 4 and in particular from 1 to 3. The catalyst beds are preferablyflowed through radially or axially by the reaction gas. In general, sucha tray reactor is operated with a fixed catalyst bed. In the simplestcase, the fixed catalyst beds are disposed axially in a shaft furnacereactor or in the annular gaps of concentric cylindrical grids. A shaftfurnace reactor corresponds to one tray. The performance of thedehydrogenation in a single shaft furnace reactor corresponds to apreferred embodiment, in which it is possible to work with oxygenouscofeed. In a further preferred embodiment, the dehydrogenation iscarried out in a tray reactor having 3 catalyst beds. In a methodwithout oxygenous gas as cofeed, the reaction gas mixture is subjectedto intermediate heating in the tray reactor on its way from one catalystbed to the next catalyst bed, for example by passing it over heatexchanger plates heated by hot gases or by passing it through tubesheated by hot combustion gases.

In a preferred embodiment of the process according to the invention, thenonoxidative catalytic n-butane dehydrogenation is carried outautothermally. To this end, the reaction gas mixture of the n-butanedehydrogenation is additionally admixed with oxygen in at least onereaction zone and the hydrogen and/or hydrocarbon present in thereaction gas mixture is at least partially combusted, which generatesdirectly in the reaction gas mixture at least a portion of the heatrequired for dehydrogenation in the at least one reaction zone.

In general, the amount of oxygenous gas added to the reaction gasmixture is selected in such a way that the amount of heat required forthe dehydrogenation of n-butane is generated by the combustion of thehydrogen present in the reaction gas mixture and of any hydrocarbonspresent in the reaction gas mixture and/or of carbon present in the formof coke. In general, the total amount of oxygen supplied, based on thetotal amount of butane, is from 0.001 to 0.5 mol/mol, preferably from0.005 to 0.2 mol/mol, more preferably from 0.05 to 0.2 mol/mol. Oxygenmay be used either as pure oxygen or as an oxygenous gas in a mixturewith inert gases, for example in the form of air. The oxygenous gascomprises preferably at least 70% by volume, more preferably at least95% by volume, of oxygen, in order to minimize the inert gas fraction inthe product gas stream of the autothermal dehydrogenation. The inertgases and the resulting combustion gases, though, have an additionaldiluting action and thus promote the heterogeneously catalyzeddehydrogenation.

The hydrogen combusted to generate heat is the hydrogen formed in thecatalytic n-butane dehydrogenation and also any hydrogen additionallyadded to the reaction gas mixture as hydrogenous gas. The amount ofhydrogen present should preferably be such that the molar H₂/O₂ ratio inthe reaction gas mixture immediately after the oxygen has been fed in isfrom 1 to 10 mol/mol, preferably from 2 to 5 mol/mol. In multistagereactors, this applies to every intermediate feed of oxygenous and anyhydrogenous gas.

The hydrogen is combusted catalytically. The dehydrogenation catalystused generally also catalyzes the combustion of the hydrocarbons and ofhydrogen with oxygen, so that in principle no specialized oxidationcatalyst is required apart from it. One embodiment works in the presenceof one or more oxidation catalysts which selectively catalyze thecombustion of hydrogen with oxygen in the presence of hydrocarbons. Thecombustion of these hydrocarbons with oxygen to give CO₂ and watertherefore proceeds only to a minor extent. The dehydrogenation catalystand the oxidation catalyst are preferably present in different reactionzones.

When the reaction is carried out in more than one stage, the oxidationcatalyst may be present only in one or in more than one reaction zone,or in all reaction zones.

Preference is given to disposing the catalyst which selectivelycatalyzes the oxidation of hydrogen at the points where there are higherpartial oxygen pressures than at other points in the reactor, inparticular near the feed point for the oxygenous gas. The oxygenous gasand/or hydrogenous gas may be fed in at one or more points in thereactor.

In one embodiment of the process according to the invention, there isintermediate feeding of oxygenous gas and, if appropriate, ofhydrogenous gas upstream of every tray of a tray reactor. In a furtherembodiment of the process according to the invention, oxygenous gas and,if appropriate, hydrogenous gas are fed in upstream of every tray exceptthe first tray. In one embodiment, a layer of a specialized oxidationcatalyst is present downstream of every feed point, followed by a layerof the dehydrogenation catalyst. In a further embodiment, no specializedoxidation catalyst is present. The dehydrogenation temperature isgenerally from 400 to 1100° C.; the pressure in the last catalyst bed ofthe tray reactor is generally from 0.2 to 5 bar, preferably from 1 to 3bar. The GHSV is generally from 500 to 2000 h⁻¹, and in high-loadoperation, even up to 100 000 h⁻¹, preferably from 4000 to 16 000 h⁻¹.

A preferred catalyst which selectively catalyzes the combustion ofhydrogen comprises oxides and/or phosphates selected from the groupconsisting of the oxides and/or phosphates or germanium, tin, lead,arsenic, antimony and bismuth. A further preferred catalyst whichcatalyzes the combustion of hydrogen comprises a noble metal oftransition group VII and/or I of the periodic table.

The dehydrogenation catalysts used generally comprise a support and anactive composition. The support generally consists of a heat-resistantoxide or mixed oxide. The dehydrogenation catalysts preferably comprisea metal oxide selected from the group consisting of zirconium dioxide,zinc oxide, aluminum oxide, silicon dioxide, titanium dioxide, magnesiumoxide, lanthanum oxide, cerium oxide and mixtures thereof, as a support.The mixtures may be physical mixtures or else chemical mixed phases suchas magnesium aluminum oxide or zinc aluminum oxide mixed oxides.Preferred supports are zirconium dioxide and/or silicon dioxide, andparticular preference is given to mixtures of zirconium dioxide andsilicon dioxide.

The active compositions of the dehydrogenation catalysts generallycomprise one or more elements of transition group VII of the periodictable, preferably platinum and/or palladium, more preferably platinum.Furthermore, the dehydrogenation catalysts may comprise one or moreelements of main group I and/or II, preferably potassium and/or cesium.The dehydrogenation catalysts may further comprise one or more elementsof transition group III including the lanthanides and actinides,preferably lanthanum and/or cerium. Finally, the dehydrogenationcatalysts may comprise one or more elements of main group III and/or IV,preferably one or more elements selected from the group consisting ofboron, gallium, silicon, germanium, tin and lead, more preferably tin.

In a preferred embodiment, the dehydrogenation catalyst comprises atleast one element of transition group VIII, at least one element of maingroup I and/or II, at least one element of main group III and/or IV andat least one element of transition group III including the lanthanidesand actinides.

For example, all dehydrogenation catalysts which are disclosed in WO99/46039, U.S. Pat. No. 4,788,371, EP-A 705 136, WO 99/29420, U.S. Pat.No. 5,220,091, U.S. Pat. No. 5,430,220, U.S. Pat. No. 5,877,369, EP 0117 146, DE-A 199 37 106, DE-A 199 37 105 and DE-A 199 37 107 may beused in accordance with the invention. Particularly preferred catalystsfor the above-described variants of the autothermal n-butanedehydrogenation are the catalysts according to examples 1, 2, 3 and 4 ofDE-A 199 37 107.

Preference is given to carrying out the n-butane dehydrogenation in thepresence of steam. The added steam serves as a heat carrier and supportsthe gasification of organic deposits on the catalysts, which counteractscarbonization of the catalysts and increases the lifetime of thecatalysts. The organic deposits are converted to carbon monoxide, carbondioxide and in some cases water.

The dehydrogenation catalyst may be regenerated in a manner known perse. For instance, steam may be added to the reaction gas mixture or agas comprising oxygen may be passed from time to time over the catalystbed at elevated temperature and the deposited carbon burnt off. Dilutionwith steam shifts the equilibrium toward the products ofdehydrogenation. After the regeneration, the catalyst is, ifappropriate, reduced with a hydrogenous gas.

The nonoxidative catalytic n-butane dehydrogenation affords a gasmixture which, in addition to butadiene, 1-butene, 2-butene andunconverted n-butane, comprises secondary constituents. Customarysecondary constituents are hydrogen, steam, nitrogen, CO and CO₂,methane, ethane, ethene, propane and propene. The composition of the gasmixture leaving the first dehydrogenation zone can vary greatlydepending on the method of dehydrogenation. For instance, when thepreferred autothermal dehydrogenation with feeding of oxygen andadditional hydrogen is carried out, the product gas mixture comprises acomparatively high content of steam and carbon oxides. In methodswithout feeding of oxygen, the product gas mixture of the nonoxidativedehydrogenation has a comparatively high content of hydrogen.

The product gas stream of the nonoxidative autothermal n-butanedehydrogenation typically contains from 0.1 to 15% by volume ofbutadiene, from 1 to 20% by volume of 1-butene, from 1 to 40% by volumeof 2-butene (cis/trans-2-butene), from 20 to 70% by volume of n-butane,from 1 to 70% by volume of steam, from 0 to 10% by volume of low-boilinghydrocarbons (methane, ethane, ethene, propane and propene), from 0.1 to40% by volume of hydrogen, from 0 to 70% by volume of nitrogen and from0 to 10% by volume of carbon dioxide.

The product gas stream b leaving the first dehydrogenation zone can beseparated into two substreams, in which case only one of the twosubstreams is subjected to the further process parts C to H and thesecond substream is recycled into the first dehydrogenation zone. Anappropriate procedure is described in DE-A 102 11 275. However, it isalso possible to subject the entire product gas stream b of thenonoxidative catalytic n-butane dehydrogenation to the further processparts C to H.

According to the invention, the nonoxidative catalytic dehydrogenationis followed downstream by an oxidative dehydrogenation(oxydehydrogenation) as process part C. This essentially dehydrogenates1-butene and 2-butene to 1,3-butadiene, and 1-butene is generallyvirtually fully depleted.

This may in principle be carried out in all reactor types and methodsknown from the prior art, for example in a fluidized bed, in a trayfurnace, in a fixed bed tubular or tube bundle reactor, or in a plateheat exchanger reactor. To carry out the oxidative dehydrogenation, agas mixture is required which has a molar oxygen:n-butenes ratio of atleast 0.5. Preference is given to working at an oxygen:n-butenes ratioof from 0.55 to 50. To attain this value, the product gas mixturestemming from the nonoxidative catalytic dehydrogenation is mixed withpure oxygen or an oxygenous gas. In the case of the first (autothermal)dehydrogenation stage B), the oxygenous gas may be air or comprisepredominantly oxygen, generally at least 70% by volume, preferably atleast 95% by volume, in order to minimize the inert gas fraction in theproduct gas stream of the oxydehydrogenation. Preference is given tooxygen of technical-grade purity. This typically comprises at least 99%by volume of oxygen. The resulting oxygenous gas mixture is then sent tothe oxydehydrogenation.

The catalysts which are particularly suitable for the oxydehydrogenationare generally based on an Mo—Bi—O multimetal oxide system whichgenerally additionally comprises iron. In general, the catalyst systemalso comprises additional components from groups 1 to 15 of the periodictable, for example potassium, magnesium, zirconium, chromium, nickel,cobalt, cadmium, tin, lead, germanium, lanthanum, manganese, tungsten,phosphorus, cerium, aluminum or silicon.

Suitable catalysts and their preparation are described, for example, inU.S. Pat. No. 4,423,281 (Mo₁₂BiNi₈Pb_(0.5)Cr₃K_(0.2)O andMo₁₂Bi_(b)Ni₇Al₃Cr_(0.5)K_(0.5)O_(x)), U.S. Pat. No. 4,336,409(Mo₁₂BiNi₆Cd₂Cr₃P_(0.5)O_(x)), DE-A 26 00 128(Mo₁₂BiNi_(0.5)Cr₃PO_(0.5)Mg_(7.5)K_(0.1)O_(x)+SiO₂) and DE-A 24 40 329(Mo₁₂BiCO_(4.5)Ni_(2.5)Cr₃P_(0.5)K_(0.1)O_(x)).

The stoichiometry of the active composition of a multitude of multimetaloxide catalysts suitable for the oxydehydrogenation can be encompassedunder the general formula (I)Mo₁₂Bi_(a)Fe_(b)Co_(c)Ni_(d)Cr_(e)X¹ _(f)K_(g)O_(x)  (I)in which the variables are each defined as follows:

X¹ = W, Sn, Mn, La, Ce, Ge, Ti, Zr, Hf, Nb, P, Si, Sb, Al, Cd and/or Mg;a = from 0.5 to 5, preferably from 0.5 to 2; b = from 0 to 5, preferablyfrom 2 to 4; c = from 0 to 10, preferably from 3 to 10; d = from 0 to10; e = from 0 to lot preferably from 0.1 to 4; f = from 0 to 5,preferably from 0.1 to 2; g = from 0 to 2, preferably from 0.01 to 1;and x = a number which is determined by the valency and frequency of theelements in (I) other than oxygen.

In the process according to the invention, preference is given to usingan Mo—Bi—Fe—O multimetal oxide system for the oxydehydrogenation,particular preference being given to an Mo—Bi—Fe—Cr—O or Mo—Bi—Fe—Zr—Omultimetal oxide system. Preferred systems are described, for example,in U.S. Pat. No. 4,547,615 (Mo₁₂BiFe_(0.1)Ni₈ZrCr₃K_(0.2)O_(x) andMo₁₂BiFe_(0.1)Ni₈AlCr₃K_(0.2)O_(x)) U.S. Pat. No. 4,424,141(Mo₁₂BiFe₃CO_(4.5)Ni_(2.5)P_(0.5)K_(0.1)O_(x)+SiO₂), DE-A 25 30 959(Mo₁₂BiFe₃CO_(4.5)Ni_(2.5)Cr_(0.5)K_(0.1)O_(x),Mo_(13.75)BiFe₃CO_(4.5)Ni_(2.5)Ge_(0.5)K_(0.8)O_(x),Mo₁₂BiFe₃CO_(4.5)Ni_(2.5)Mn_(0.5)K_(0.1)O_(x) andMo₁₂BiFe₃CO_(4.5)Ni_(2.5)La_(0.5)K_(0.1)O_(x)), U.S. Pat. No. 3,911,039(Mo₁₂BiFe₃CO_(4.5)Ni_(2.5)Sn_(0.5)K_(0.1)O_(x)), DE-A 25 30 959 and DE-A24 47 825 (Mo₁₂BiFe₃CO_(4.5)Ni_(2.5)W_(0.5)K_(0.1)O_(x)). Thepreparation and characterization of the catalysts mentioned aredescribed comprehensively in the documents cited.

The oxydehydrogenation catalyst is generally used in the form of shapedbodies having an average size of over 2 mm. Owing to the pressure dropto be observed when the process is performed, smaller shaped bodies aregenerally unsuitable. Examples of suitable shaped bodies includetablets, cylinders, hollow cylinders, rings, spheres, strands, wagonwheels or extrudates. Special shapes, for example “trilobes” and“tristars” (see EP-A-0 593 646) or shaped bodies having at least onenotch on the exterior (see U.S. Pat. No. 5,168,090) are likewisepossible.

In general, the catalyst used may be used in the form of an unsupportedcatalyst. In this case, the entire shaped catalyst body consists of theactive composition, including any auxiliaries, such as graphite or poreformers, and also further components. In particular, it has been foundto be advantageous to use the Mo—Bi—Fe—O catalyst used with preferencefor the oxydehydrogenation of the n-butenes to butadiene in the form ofan unsupported catalyst. Furthermore, it is possible to apply the activecompositions of the catalysts to a support, for example an inorganic,oxidic shaped body. Such catalysts are generally referred to as coatedcatalysts.

The oxydehydrogenation is generally carried out at a temperature of from220 to 490° C. and preferably from 250 to 450° C. A reactor inletpressure is selected which is sufficient to overcome the flowresistances in the plant and the subsequent workup. This reactor inletpressure is generally from 0.005 to 1 MPa gauge, preferably from 0.01 to0.5 MPa gauge. By its nature, the gas pressure applied in the inletregion of the reactor decreases substantially over the entire catalystbed.

The coupling of the nonoxidative catalytic, preferably autothermal,dehydrogenation with the oxidative dehydrogenation of the n-butenesformed affords a very much higher yield of butadiene based on n-butaneused. The nonoxidative dehydrogenation can also be operated in a gentlermanner. Comparable butadiene yields would only be achievable with anexclusively nonoxidative dehydrogenation at the cost of distinctlyreduced selectivities. An exclusively oxidative dehydrogenation onlyachieves low n-butane conversions.

In addition to butadiene and unconverted n-butane, the product gasstream c leaving the oxidative dehydrogenation also comprises hydrogen,carbon dioxide and steam. As secondary constituents, it can alsocomprise oxygen, nitrogen, methane, ethane, ethene, propane and propene,and also oxygenous hydrocarbons, known as oxygenates. In general, itcomprises virtually no 1-butene and only small fractions of 2-butene.

In general, the product gas stream c leaving the oxidativedehydrogenation has from 1 to 40% by volume of butadiene, from 1 to 80%by volume of n-butane, from 0 to 5% by volume of 2-butene, from 0 to 1%by volume of 1-butene, from 5 to 70% by volume of steam, from 0 to 10%by volume of low-boiling hydrocarbons (methane, ethane, ethene, propaneand propene), from 0.1 to 15% by volume of hydrogen, from 0 to 40% byvolume of nitrogen, from 0 to 10% by volume of carbon dioxide and from 0to 10% by volume of oxygenates. Oxygenates may, for example, be furan,acetic acid, maleic anhydride, maleic acid, propionic acid,acetaldehyde, acrolein, formaldehyde, formic acid and butyraldehyde. Inaddition, traces of acetylene, propyne and 1,2-butadiene may also bepresent.

The product gas stream c may also comprise small amounts of oxygen. Whenthe product gas stream c contains more than just minor traces of oxygen,a process stage is generally carried out to remove residual oxygen fromthe product gas stream c. The residual oxygen may have a troublesomeeffect insofar as it can act as an initiator for polymerizationreactions in downstream process steps. This is a risk especially in thecourse of the distillative removal of butadiene (step E)) and can leadthere to deposits of polymers (formation of “popcorn”) in the extractivedistillation column. Preference is given to carrying out the oxygenremoval immediately after the oxidative dehydrogenation. To this end, acatalytic combustion stage is generally carried out in which oxygen isreacted with the hydrogen present in the gas stream c in the presence ofa catalyst. This achieves a reduction in the oxygen content down tosmall traces.

A suitable catalyst for the oxidation of hydrogen comprises, supportedon α-alumina, from 0.01 to 0.1% by weight of platinum and from 0.01 to0.1% by weight of tin, based on the total weight of the catalyst.Platinum and tin are used advantageously in a weight ratio of from 1:4to 1:0.2, preferably in a ratio of from 1:2 to 1:0.5, in particular in aratio of approximately 1:1. Advantageously, the catalyst comprises from0.05 to 0.09% by weight of platinum and from 0.05 to 0.09% by weight oftin, based on the total weight of the catalyst. In addition to platinumand tin, alkali metal and/or alkaline earth metal compounds may ifappropriate be used in amounts of less than 2% by weight, in particularless than 0.5% by weight. More preferably, the alumina catalystcomprises exclusively platinum and tin. The catalyst support ofα-alumina advantageously has a BET surface area of from 0.5 to 15 m²/g,preferably from 2 to 14 m²/g, in particular from 7 to 11 m²/g. Thesupport used is preferably a shaped body. Preferred geometries are, forexample, tablets, annular tablets, spheres, cylinders, star extrudatesor toothed wheel-shaped extrudates having diameters of from 1 to 10 mm,preferably from 2 to 6 mm. Particular preference is given to spheres orcylinders, in particular cylinders.

Alternative processes for removing residual oxygen from the product gasstream c comprise the contacting of the product gas stream with amixture of metal oxides which comprise copper in reduced form in the 0oxidation state. In addition, such a mixture generally also comprisesaluminum oxides and zinc oxides, the copper content being typically upto 10% by weight. In this way, virtually full conversion of residualoxygen is possible. In addition, further methods of removing oxygentraces may be used. Examples are the removal by means of molecularsieves or use of membranes.

In one process stage, D), the gas stream c is compressed in at least onefirst compression stage and subsequently cooled, in the course of whichat least one condensate stream d1 comprising water condenses out and thegas stream d2 comprising n-butane, butadiene, hydrogen, carbon dioxideand steam remains.

Preferably, the gas stream c is cooled to a temperature in the rangefrom 15 to 60° C. before the first compression stage. Cooling is carriedout via direct or indirect heat exchange. For direct heat exchange,recycled condensate is brought into contact with gas stream c. Suitablecontact apparatuses are wash columns, quench columns, venturi washers.Optionally, NaNO₂ is added to the quench cycle stream to remove tracesof oxygen. Optionally, stabilizer against the formation of popcorn,polymers or butadiene epoxides is added to the quench cycle stream.

The compression may be effected in one or more stages. Overall,compression is effected from a pressure in the range from 1.0 to 4.0 barto a pressure in the range from 3.5 to 8.0 bar. Each compression stageis followed by a cooling stage in which the gas stream is cooled to atemperature in the range from 15 to 60° C. The condensate stream d1 maythus also comprise a plurality of streams in the case of multistagecompression.

The gas stream d2 consists generally substantially of C₄ hydrocarbons(substantially n-butane and butadiene), hydrogen, carbon dioxide andsteam. In addition, the stream d2 may also comprise low boilers andinert gases (nitrogen) as further secondary components. The wastewaterstream d1 consists generally to an extent of at least 80% by weight,preferably to an extent of at least 90% by weight, of water andcomprises additionally, to a small extent, low boilers, C₄ hydrocarbons,oxygenates and carbon dioxide.

Suitable compressors are, for example, turbo compressors and pistoncompressors including rotary piston compressors. The compressors may bedriven, for example, by an electric motor, an expander or a gas or steamturbine. Typical compression ratios (outlet pressure:iniet pressure) fora compressor stage, depending on the design, are between 1.5 and 3.0.

The compressed gas is cooled by heat exchangers which may be designed,for example, as tube bundle, spiral or plate heat exchangers. Thecoolants used in the heat exchangers are cooling water or heat carrieroils. In addition, preference is given to using air cooling with use offans.

In one process stage, E), the gas stream d2 is separated by extractivedistillation into a product stream e1 consisting substantially ofbutadiene and a stream e2 comprising n-butane, hydrogen, carbon dioxideand steam.

The extractive distillation may be carried out, for example, asdescribed in Erdöl und Kohle—Erdgas—Petrochemie [Mineral Oil andCoal—Natural Gas—Petrochemistry] volume 34 (8), pages 343-346 orUllmanns Enzyklopädie der Technischen Chemie, volume 9, 4th edition1975, pages 1 to 18.

To this end, the gas stream d2 is contacted in an extraction zone withan extractant, preferably an N-methylpyrrolidone (NMP)/water mixture.The extraction zone is generally configured in the form of a wash columnwhich comprises trays, random packings or structured packings asinternals. It generally has from 30 to 70 theoretical plates, so thatsufficiently good separating action is achieved. The wash columnpreferably has a backwash zone in the top of the column. This backwashzone serves to recycle the extractant present in the gas phase by meansof a liquid hydrocarbon reflux, for which the top fraction is condensedbeforehand. Typical temperatures at the top of the column are between 30and 60° C. The mass ratio of extractant to C₄ product gas stream d2 inthe feed of the extraction zone is generally from 10:1 to 20:1.

Suitable extractants are butyrolactone, nitrites such as acetonitrile,propionitrile, methoxypropionitrile, ketones such as acetone, furfural,N-alkyl-substituted lower aliphatic amides such as dimethylformamide,diethylformamide, dimethylacetamide, diethylacetamide,N-formylmorpholine, N-alkyl-substituted cyclic amides (lactams) such asN-alkylpyrrolidones, especially N-methylpyrrolidone (NMP). In general,alkyl-substituted lower aliphatic amides or N-alkyl-substituted cyclicamides are used. Particularly advantageous are dimethylformamide,acetonitrile, furfural and especially NMP.

However, it is also possible to use mixtures of these extractants withone another, for example of NMP and acetonitrile, mixtures of theseextractants with cosolvents and/or tert-butyl ethers, e.g. methyltert-butyl ether, ethyl tert-butyl ether, propyl tert-butyl ether, n- orisobutyl tert-butyl ether. Particularly suitable is NMP, preferably inaqueous solution, preferably with from 0 to 20% by weight of water, morepreferably with from 7 to 10% by weight of water, in particular with8.3% by weight of water.

In the extractive distillation column, a gaseous stream e2 comprisingn-butane, steam, hydrogen and carbon dioxide is obtained, which isgenerally drawn off via the top of the column, and the side draw streamobtained is a mixture of extractant and butadiene. From this mixture,butadiene may be obtained subsequently as a pure product. The extractantwhich also comprises butadiene and any secondary components (impurities)is obtained as a side draw stream. The side draw stream is, ifappropriate after carrying out further purification steps, recycled backinto the extractive distillation.

The stream e2 may comprise, as further constituents, also butenes, lowboilers and inert gases (nitrogen).

For example, the extractive distillation, isolation of the purebutadiene and purification of the extractant may be carried out asfollows: the side draw stream of the extractive distillation column,composed of extractant and butadiene which still comprises impurities(acetylene, propyne, 1,2-butadiene), is fed into a wash column which ischarged with fresh extractant. At the top of the wash column, crudebutadiene which comprises, for example, 98% by weight of butadiene isdrawn off. The bottom draw stream is enriched with acetylene and isrecycled into the extractive distillation. The crude butadiene maycomprise propyne and 1,2-butadiene as impurities. To remove theseimpurities, the crude butadiene is fed to a first purifying distillationcolumn and a propyne-enriched butadiene stream is removed overhead. Thebottom draw stream which is substantially propyne-free, but stillcontains traces of 1,2-butadiene, is fed into a second purifyingdistillation column in which a substantially 1,2-butadiene-free purebutadiene stream having a purity of, for example, at least 99.6% byweight as a top draw stream or side draw stream in the rectifyingsection of the column, and a 1,2-butadiene-enriched bottom draw stream,are obtained.

To purify the extractant, a portion of the extractant is discharged fromthe extractive distillation column as a bottom draw stream andregenerated as follows: the extraction solution is transferred into adesorption zone with reduced pressure and/or elevated temperaturecompared to the extraction zone, and butadiene and acetylene tracespresent are desorbed from the extraction solution. The desorption zonemay be designed, for example, in the form of a wash column which hasfrom 5 to 15, preferably from 8 to 10, theoretical plates and a backwashzone having, for example, 4 theoretical plates. This backwash zoneserves to recover the extractant present in the gas phase by means ofliquid hydrocarbon recycling, for which the top fraction is condensedbeforehand. The internals provided are structured packings, trays orrandom packings. The pressure added to the top of the column is, forexample, 1.5 bar. The temperature in the bottom of the column is, forexample, from 130 to 150° C. At the bottom of the column, asubstantially acetylene-free extractant is obtained and is recycled intothe extractive distillation column.

The product-of-value stream e1, as is obtained, for example, at the topdraw stream of the second purifying distillation column, may comprise upto 100% by volume of butadiene.

The extraction solution is transferred into a desorption zone havingreduced pressure and/or elevated temperature compared to the extractionzone, and the butadiene is desorbed from the extraction solution. Thedesorption zone may be designed, for example, in the form of a washcolumn which has from 5 to 15, preferably from 8 to 10, theoreticalplates, and a backwash zone having, for example, 4 theoretical plates.This backwash zone serves to recover the extractant present in the gasphase by means of liquid hydrocarbon reflux, for which the top fractionis condensed beforehand. The internals provided are structured packings,trays or random packings. The pressure at the top of the column is, forexample, 1.5 bar. The temperature in the bottom of the column is, forexample, from 130 to 150° C.

In one process stage, F), the gas stream e2 is compressed in at leastone further compression stage and subsequently cooled to obtain at leastone condensate stream f1 comprising n-butane and water, a gas stream f2comprising n-butane, hydrogen and carbon dioxide.

The compression may again be effected in one or more stages. In general,compression is effected from a pressure in the range from 3.5 to 8 barto a pressure in the range from 12 to 40 bar. Each compression stage isfollowed by a cooling stage in which the gas stream is cooled to atemperature in the range from 15 to 60° C. The condensate stream f1 maythus also comprise a plurality of streams in the case of multistagecompression.

Preference is given to carrying out the compression in two stages,compression being effected in a first compression stage to a pressure offrom 8 to 15 bar and in a second compression stage to a pressure of from12 to 40 bar. After each compression stage there is a cooling stage(intermediate cooling), in each case to obtain one condensate stream.These are fed together or separately to the phase separation.

The gas stream f2 comprises generally n-butane, carbon dioxide andhydrogen as the essential components. In addition, it may also comprisebutenes, low boilers and inert gases (nitrogen) as further secondarycomponents. Steam may also be present in small amounts. The condensatestream f1 consists generally to an extent of at least 40% by weight,preferably to an extent of at least 60% by weight, of C₄ hydrocarbons(substantially n-butane, additionally in some cases also butenes) andcomprises additionally water and generally carbon dioxide; it mayfurther comprise low boilers and oxygenates.

In one process stage, G), the gas stream f2 is cooled to obtain acondensate stream g1 comprising n-butane and an offgas stream g2comprising carbon dioxide and hydrogen. The condensation may be carriedout in more than one stage, for example in two stages or asrectification with 5 to 15 theoretical stages. In this case, a pluralityof condensate streams g1 may be obtained.

Before the condensation stage G) is carried out, small amounts of steammay be removed from the gas stream f2 by absorption on a molecularsieve. This is necessary at condensation temperatures of ≦0° C. in orderto avoid the freezing out of water.

The condensate stream g1 may, for example, be condensed in surfacecondensers. In these condensers, the gas stream f2 comes into contactwith tubes which are flowed through by cooling medium. Useful coolantsare, for example, water, air and cooling brine. Injection condensers inwhich the coolant, preferably water, is injected directly into the gasstream which comprises the components to be condensed are also possible.

In general, the gas stream f2 is cooled to a temperature in the rangefrom −30 to +20° C. The condensate stream g1 comprises predominantly C₄hydrocarbons (substantially n-butane and in some cases butenes),generally to an extent of at least 50% by weight, preferably to anextent of at least 70% by weight, and generally additionally comprisescarbon dioxide. In addition, it may also comprise low boilers and smallamounts of water.

In one process step, H), water is removed by phase separation from theat least one condensate stream f1 and, if appropriate, the condensatestream g1, to obtain at least one recycle stream h1 comprising n-butaneand at least one wastewater stream h2, and the at least one recyclestream h1 is recycled into the first dehydrogenation zone.

To this end, the condensate streams f1 and, if appropriate, g1 are fedseparately or combined to one or more phase separation apparatuses. Thecondensate stream g1 is subjected to a phase separation if it still hasa significant water content, otherwise it can be recycled directly intothe first dehydrogenation zone. A combined C₄ hydrocarbon stream h1 or aplurality of separate C₄ hydrocarbon streams h1 is/are obtained. Thehydrocarbon stream(s) h1 comprise(s) predominantly C₄ hydrocarbons(substantially n-butane, additionally in some cases butenes), generallyto an extent of at least 70% by weight, preferably to an extent of atleast 80% by weight, and may additionally comprise carbon dioxide, lowboilers and traces of water. The wastewater stream h2 comprisespredominantly water, generally to an extent of at least 65% by weight,and generally additionally comprises carbon dioxide. In addition,hydrocarbons (low boilers and C₄ hydrocarbons) may also be present.

The hydrocarbon streams h1 and, if appropriate, the stream g1 removed byphase separation are recycled partly or fully into the firstdehydrogenation zone.

The offgas stream g2 generally comprises predominantly carbon dioxideand in some cases inert gases, additionally hydrogen, C₄ hydrocarbons toa small extent and in some cases low boilers.

The phase separation is preferably effected under gravity in ahorizontal or vertical phase separator. The phase separator may comprisesedimentation aids (for example random packings or plates) or coalescentfilters (for example of fiber material).

To remove the hydrogen present in the offgas stream g2, it may, ifappropriate on completion of cooling, for example in an indirect heatexchanger, be passed through a membrane, generally configured as a tube,which is permeable only to molecular hydrogen. The thus removedmolecular hydrogen may, if required, be used at least partly in thedehydrogenation or else sent to another utilization, for example forgenerating electrical energy in fuel cells.

EXAMPLE

A feed gas stream (4) comprising n-butane, said stream being obtained bycombining a fresh gas stream (1) and a recycle stream (15), is fed tothe first, autothermally operated, nonoxidative catalytic n-butanedehydrogenation stage (BDH) (18). To provide the heat required for theendothermic dehydrogenation, hydrogen is combusted selectively. To thisend, combustion air is fed as stream (2). In order to counteractcarbonization of the catalyst and prolong the lifetime of the catalyst,steam (3) is also added. A dehydrogenation gas mixture (5) is obtained,which is cooled after leaving the autothermal dehydrogenation stage (18)and fed to the second, oxidative, n-butane dehydrogenation stage (ODH)(19). Also fed to the second dehydrogenation stage (19) is an oxygenstream (6). For BDH and ODH, based on experimental results, the degreesof conversion and selectivities reproduced in Table 1 were assumed.

TABLE 1 Reaction stage Conversion [%] Selectivity [%] Autothermal 49.597.9 dehydrogenation (BDH) (n-butane) (to butenes/butadiene) Oxidative100.0 (1-butene) 95.0 dehydrogenation (ODH)  92.7 (2-butene) (tobutadiene)

From the exit gas of the oxydehydrogenation (7), which is under apressure of 2.2 bar, the residual oxygen is removed by catalyticcombustion of hydrogen, which results in a virtually oxygen-free gasstream (7 a). To this end, the gas stream (7) is contacted with acatalyst in the reactor (20). Subsequently, the gas stream (7 a) iscooled and compressed to a pressure of 5.1 bar in a compressor (21) andcooled to a temperature of 55° C., to obtain a wastewater stream 8. Thecompressed gas (9) is cooled and fed to an extraction column (22), wherethe removal of butadiene (10) is effected using NMP as a solvent. Theexit gas of the butadiene extraction stage (22), which is under apressure of 5 bar and consists substantially of n-butane, carbon dioxideand hydrogen and additionally low boilers and steam, is compressed to apressure of 30.1 bar in two stages in two compressors (23) and (24) withintermediate cooling, and the compressed gas (14) is cooled to atemperature of 5° C. in two stages in a condenser (25). The condensate(12) is obtained at a pressure of 12 bar and a temperature of 55° C.;the condensate (14 a) is obtained at a pressure of 30.1 bar and atemperature of 55° C. The condensate streams (12), (14 a), (17 a 1) and(17 a 2) obtained in the compression/condensation comprise predominantlyn-butane and additionally carbon dioxide, water, butenes and lowboilers. The uncondensed offgas (17) is hydrogen-rich and is fed eitherto a combustion or a material utilization (pressure-swing absorption,membrane removal of the hydrogen). The condensate streams are fed to aphase separator and separated into an aqueous phase (wastewater stream16) and an organic phase (15).

The results of the simulation calculation are reported in Table 2. Thecomposition of the streams (1) to (17 a 2) is reported in parts byweight.

TABLE 2 Stream No. 1 2 3 4 5 6 7 7a 8 Amount [kg/h] 24834 4779 123159068 59067 9799 68866 68865 14685 PROPANE 0.0000 0.0000 0.0000 0.02430.0309 0.0000 0.0265 0.0265 0.0187 BUTANE 1.0000 0.0000 0.0000 0.84080.4272 0.0000 0.3665 0.3665 0.0001 1-BUTENE 0.0000 0.0000 0.0000 0.00000.1155 0.0000 0.0000 0.0000 0.0000 CIS-2-BUTENE 0.0000 0.0000 0.00000.0032 0.1174 0.0000 0.0050 0.0050 0.0094 TRANS-2-BUTENE 0.0000 0.00000.0000 0.0074 0.1471 0.0000 0.0113 0.0113 0.0201 1,3-BUTADIENE 0.00000.0000 0.0000 0.0000 0.0208 0.0000 0.3014 0.3014 0.0003 WATER 0.00000.0000 1.0000 0.0216 0.1059 0.0000 0.2052 0.2102 0.9419 CARBON DIOXIDE0.0000 0.0000 0.0000 0.0218 0.0284 0.0000 0.0729 0.0729 0.0095 HYDROGEN0.0000 0.0000 0.0000 0.0000 0.0057 0.0000 0.0049 0.0043 0.0000 OXYGEN0.0000 0.9901 0.0000 0.0801 0.0004 0.9912 0.0046 0.0000 0.0000 N2 0.00000.0099 0.0000 0.0008 0.0008 0.0088 0.0019 0.0019 0.0000 Temperature [°C.] 25 10 143.61 420 520 17.5 380 55 55 Pressure [bar] 3.2 3.2 3.2 3.22.7 2.7 2.2 2.1 5.1 Stream No. 9 10 11 12 13 14a 14 Amount [kg/h] 5418020750 33430 15971 17459 10227 7232 PROPANE 0.0286 0.0000 0.0463 0.05550.0379 0.0504 0.0203 BUTANE 0.4658 0.0000 0.7549 0.8675 0.6519 0.85590.3634 1-BUTENE 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000CIS-2-BUTENE 0.0038 0.0000 0.0062 0.0077 0.0049 0.0066 0.0024TRANS-2-BUTENE 0.0088 0.0000 0.0143 0.0173 0.0117 0.0155 0.00621,3-BUTADIENE 0.3830 1.0000 0.0000 0.0000 0.0000 0.0000 0.0000 WATER0.0119 0.0000 0.0193 0.0338 0.0060 0.0079 0.0034 CARBON DIOXIDE 0.09010.0000 0.1460 0.0182 0.2629 0.0637 0.5445 HYDROGEN 0.0055 0.0000 0.00890.0000 0.0170 0.0000 0.0410 OXYGEN 0.0001 0.0000 0.0001 0.0000 0.00020.0000 0.0004 N2 0.0025 0.0000 0.0040 0.0000 0.0076 0.0000 0.0184Temperature [° C.] 55 111.66 111.66 55 55 55 55 Pressure [bar] 5.1 5 512 12 30.1 30.1 Stream No. 15 16 17 17a1 17a2 Amount [kg/h] 28229 8134387 1834 1011 PROPANE 0.05080 0.07185 0.01273 0.03283 0.03015 BUTANE0.87976 0.00501 0.09033 0.81808 0.72345 1-BUTENE 0.00000 0.00000 0.000000.00000 0.00000 CIS-2-BUTENE 0.00678 0.01858 0.00046 0.00595 0.00469TRANS-2-BUTENE 0.01556 0.04243 0.00132 0.01464 0.01212 1,3-BUTADIENE0.00000 0.00000 0.00000 0.00000 0.00000 WATER 0.00151 0.74052 0.000130.01031 0.00476 CARBON DIOXIDE 0.04559 0.12157 0.79644 0.11815 0.22475HYDROGEN 0.00000 0.00001 0.06754 0.00001 0.00001 OXYGEN 0.00000 0.000000.00070 0.00000 0.00001 N2 0.00001 0.00003 0.03035 0.00003 0.00008Temperature [° C.] 30 30 5 30 5 Pressure [bar] 30.1 30.1 30.1 30.1 30.1

1. A process for preparing butadiene from n-butane, comprising the stepsof A) providing a feed gas stream (a) comprising n-butane; B) feedingsaid feed gas stream (a) into at least one first dehydrogenation zoneand nonoxidatively, catalytically dehydrogenating n-butane to obtain agas stream (b) comprising n-butane, 1-butene, 2-butene, butadiene, andhydrogen, and optionally comprising carbon dioxide and/or steam; C)feeding said gas stream (b) and an oxygenous gas into at least onesecond dehydrogenation zone and oxidatively dehydrogenating 1-butene and2-butene to obtain a gas stream (c) comprising n-butane, butadiene,hydrogen, carbon dioxide, and steam; D) compressing in at least onefirst compression stage and cooling said gas stream (c) to obtain atleast one condensate stream (d1) comprising water, and a gas stream (d2)comprising n-butane, butadiene, hydrogen, carbon dioxide, and steam; E)separating said gas stream (d2) by extractive distillation into aproduct stream (e1) consisting substantially of butadiene, and a stream(e2) comprising n-butane, hydrogen, carbon dioxide, and steam, F)compressing in at least one further compression stage and cooling saidgas stream (e2) to obtain at least one condensate stream (f1) comprisingn-butane and water, and a gas stream (f2) comprising n-butane, hydrogen,and carbon dioxide, G) cooling said gas stream (f2) to obtain acondensate stream (g1) comprising n-butane, and an offgas stream (g2)comprising carbon dioxide and hydrogen, and H) removing water from saidat least one condensate stream (f1), and optionally from said condensatestream (g1), by phase separation to obtain at least one recycle stream(h1) comprising n-butane, and at least one wastewater stream (h2), andrecycling said at least one recycle stream (h1) into said firstdehydrogenation zone.
 2. The process according to claim 1, wherein thenonoxidative, catalytic dehydrogenation of n-butane of step B) iscarried out autothermally while feeding in an oxygenous gas.
 3. Theprocess according to claim 2, wherein said oxygenous gas is air.
 4. Theprocess according to claim 2, wherein said oxygenous gas is oxygen oftechnical-grade purity.
 5. The process according to claim 1, whereinsaid feed stream is obtained from liquefied petroleum gas.
 6. Theprocess according to claim 1, comprising the additional step of removingthe oxygen remaining in the product gas of the oxidative dehydrogenationby reacting it catalytically with hydrogen after step C).